The structural control of impact craters: Evidence from the terrestrial planets
|Location||International Geological Congress,oslo 2008|
|Author||hman, Teemu; Aittola, Marko; Kostama, Veli-Petri; Raitala, Jouko|
|Holding Date||08 October 2008|
Impact craters are not merely circular holes in the ground. Contrary to the common misconception, impact crater formation is sometimes controlled by pre-existing structures (faults, joint sets, etc.) in the target [e.g. 1-5]. Such structural control leads to the formation of polygonal impact craters (PICs), which are commonly seen on all the terrestrial planets, numerous moons, asteroids and a cometary nucleus. On Venus  and Argyre region, Mars, PICs make ~13%-22% of the crater population.
PICs are an important tectonic indicator. The Argyre region evidences that the PICs reflect a tectonic system older than the Tharsis-controlled ridges and graben. This may be related to an early phase of volcanotectonism in eastern Thaumasia. However, the most dominant straight PIC rim orientations emerge where two (or more) large-scale fracture orientations in the target coincide. Such orientations in the Argyre-Ladon and Hellas-Isidis basin regions are the dominant basin-induced radial and less significant concentric fractures, as well as the possible conjugate shear fractures. A re-examination of our Hellas region PIC data  also strengthens the idea of a large ancient impact basin south of Hephaestus Fossae .
The compiled PIC diameter data from the Argyre region and Venus  suggests that there is a "preferred" size range for PIC formation with respect to the simple-to-complex transition diameter (Dtr). Based on data from Mars and Venus, this size seems to be about 2-5Dtr. Interestingly, Pohn and Offield  classified lunar craters 20-45 km in diameter (~1.3-3Dtr) as markedly more polygonal than smaller or larger craters. Our preliminary results from the lunar near-side give tentative support to this. Hence, it appears there is a specific size range (a few times Dtr) that favours the formation of polygonal craters, regardless of the target properties or gravity.
In addition to the "enhanced excavation" and "structurally controlled slumping" -models of PIC formation [2-3], we suggest a third mechanism, namely thrusting along planes of weakness in the excavation stage. Thrusts are the dominant structural features in terrestrial simple (e.g. Meteor & Tswaing) and small complex (Bosumtwi) crater rims. This is also the size range of our proposed thrusting mechanism for PIC formation. In complex craters this is fascinatingly the same size as the apparent "favourable" PIC formation size. Larger polygonal craters are most likely dominated by the structurally controlled slumping.
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